Colour Display Reversed

This sounds silly, but our cat ran across our Mac Book and in doing so he has reversed the colour display, so that now the desktop and every programme's colour is in reverse, like a photographic negative.

Searching Help has been no use at all - does anyone know how to resolve this?

- Too many "beachballs", read this: Beachballs- Computer seems slower than it used to? Read this for some slow computer tips: Speedup- Almost full hard drive? Some solutions. Out of Space- Apple Battery Info. Battery